As wgstarks said, allocation method controls where files are written, but after written unRAID will not move them. If you want to move them yourself the unBalance plugin might help.
Okey, Thanks wgstarks and trurl.
I see I have the standard "High-Water" option chosen.
But I will buy 1x new 4TB and remove my 2x 500GB drives, is the best way to do this by shrinking the array (removing the 2x 500GB disks) first and then add my new 4TB as normal? http://lime-technology.com/wiki/index.php/Shrink_array
This one might be more appropriate:
Replacing Multiple Data Drives with a Single Larger Drive